Patients with leukemia or lymphoma on chemotherapy oftentimes present with CNS symptoms. CT scan is usually negative at the early stages. Late sequelae of CNS damage appear on CT many months or years after the treatment. This study is undertaken to demonstrate the early brain lesions by MRI. Preliminary results indicated that precursors of permanent brain lesions can be demonstrated by MRI. By knowing the natural history of these lesions we will be in the position to modify the mode of treatment in order to achieve the best therapeutic results with the least possible side effects.